Anode for oxygen evolution in electrolytes containing manganese and fluorides

ABSTRACT

A new electrocatalytic coating to be applied onto a titanium matrix, suitable for oxygen evolution from acid electrolytes containing manganese and fluorides, comprising: 
     a) an external coating for oxygen evolution at controlled potential, immune to manganese electrochemical precipitation and capable of promoting the spontaneous removal of the same during operation, consisting of ruthenium and iridium as the major components (60-85%), tin and cobalt (2-10%) and titanium and tantalum at intermediate concentrations with respect to the previous groups of components. 
     b) an optional interlayer acting as an electroconductive system and protecting the titanium matrix against corrosion caused by fluorides, made of titanium and tantalum as the major components (&lt;95%) and iridium (&gt;5%) as the minor component. 
     At least part of the above elements are in the form of oxides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns electrocatalytic coatings foroxygen-evolving anodes.

2. Description of the Related Art

The anodic materials of the prior art for the electrometallurgy ofcopper, zinc and cobalt are essentially of two types: lead alloys, andcobalt-silicon alloys (cobalt only). Industrial lead anodes are made oflead alloys containing one or more elements selected in the followinggroup: I B, IV A and V A. In particular, the lead-silver (0.2-0.8%)anode is commonly used especially in the zinc electrometallurgy, whilefor the cobalt electrometallurgy different alloys are used, such aslead-antimony (2-6%), lead-silver (0.2-0.8%), lead-tin (5-10%). Thesematerials are characterized by:

high anodic potentials, above 1.9 V (NHE).

lifetimes in the range of 1 to 3 years

high electric resistivity and substantial electric disuniformity leadingto the formation of thick solid layers of PbSO₄ (intermediatepassivating layer) and PbO₂ (external electrocatalytic layer for oxygenevolution).

These characteristics involve the following drawbacks:

faradic efficiency loss (below 90% for zinc, and below 95% for cobalt)

uneven and dendritic aspect of the deposit

contamination by lead of the produced metal.

The cobalt alloys, used for a part of the cobalt electrometallurgy, aresubstantially of three types characterized by the followingcompositions: cobalt-silicon (5-20%), cobalt-silicon (5-20%)-manganese(1.0-5.0%), cobalt-silicon (5-20%)-copper (0.5-2.5%).

The cobalt-silicon alloys, with respect to the lead alloys, arecharacterized by a longer lifetime but are affected by a higherelectrical resistivity and brittleness, while the cobalt-silicon-copperalloys have a shorter lifetime and are all the same fragile.

As concerns cathode poisoning, this occurs only when copper alloys areused.

Table 1 summarizes some examples of general operating conditions of theprior art technology. Reference is made to the process for zinc andcobalt deposition.

                                      TABLE 1                                     __________________________________________________________________________                         Prior art operating materials                                                 Anode lifetime (years)                                                 Current        Co--Si                                           Process                                                                           Electrolyte                                                                             Density A/m.sup.2                                                                    Pb--Sn                                                                            Pb--Ag                                                                            Co--Si--Mn                                                                          Co--Si--Cu                                 __________________________________________________________________________    Zinc                                                                              Zn.sup.2+ 300-500 g/l)                                                                         //  2-4 //    //                                             H.sub.2 SO.sub.4 (150-200 g/l)                                                Fluorides (50 ppm)                                                            Manganese (2-8 g/l)                                                           Zn.sup.2+ 300-500 g/l)                                                                         1-3 2-4 //    //                                             H.sub.2 SO.sub.4 (150-200 g/l)                                                Fluorides (5 ppm)                                                             Manganese (2-8 g/l)                                                       Cobalt                                                                            Co.sup.2+ 160-250 g/l)                                                                         4-5 4-5 3-4   2-3                                            H.sub.2 SO.sub.4 (pH 1-3)                                                     Manganese (10-30 g/l)                                                         pH = 4-5,5                                                                __________________________________________________________________________

In the electrolysis of solutions containing, besides the salt of themetal to be deposited, also significant quantities of manganese (5-20g/l and more), two reactions take place at the anode, and precisely:

oxygen evolution: 2H₂ O=4H+O₂ +4e

manganese dioxide deposition (parasitic reaction): 2Mn²⁺ +4H₂ O=2MnO₂+8H⁺ +4e.

This anodic by-product is an electrically resistive oxide (resistivityequal or higher than that of the PbO₂ -PbSO₄ mixture formed on leadanodes); as a consequence, its precipitation on the surface of theelectrode, if compact and continuous with time, involves a progressiveincrease of the electrode potential, which negatively affects prior artelectrodes. In industrial practice, to avoid or at least control thisphenomenon, the anodes (lead alloys or cobalt alloys) are periodicallycleaned by mechanical brushing carried out outside the electrolysiscell.

It is known that titanium electrodes, activated by conventional coatingsbased on tantalum and iridium oxides, when used in electrolytescontaining manganese, are negatively affected by the same drawbacks aslead anodes, with the only difference that the mechanical cleaning isnot applicable due to the insufficient mechanical stability of thecoating. Therefore, possible alternatives to the mechanical removal ofthe MnO₂ have been considered, such as periodical washing outside thecell with reducing solutions such as H₂ O₂ ; H₂ O₂ +nitrates or nitricacid; ferrous salts and nitrates, ferrous salts and sulphates, etc. oractions carried out in the cell, such periodical current reversal,periodical current interruption, scheduled shut-downs etc. As theresults were either negative or unsuitable for industrial scaleapplication, efforts have been focused on the spontaneous removal ofmanganese dioxide electrodeposited onto the anode directly in theelectrolysis cell.

SUMMARY OF THE INVENTION

It is the main object of the present invention to provide for an anodecapable of promoting the spontaneous and continuous removal of themanganese dioxide, MnO₂, formed by the aforementioned parasiticreaction, so that the growth in thick layers is prevented.

DETAILED DESCRIPTION

The anode of the invention comprises an electrocatalytic surface coatingfor oxygen evolution applied on a titanium matrix, suitable foroperation at controlled potential. Optionally an inter-layer may beprovided, which acts as an electroconductive system for protecting thetitanium matrix (stabilizing action towards fluorides and acidity). Thefollowing complementary criteria are used for selecting the surfacecoating:

a) addition of highly catalytic metals for oxygen evolution, for exampleruthenium and cobalt, to the main components consisting of tantalum andiridium, to fix the voltage at low and controlled values.

b) further addition of metals capable of stabilizing ruthenium andcobalt, such as titanium and tin.

The invention will be better illustrated making reference to someexamples, which are not intended to limit the same.

For all of the Examples, the samples, consisting of a matrix made oftitanium grade 1, having the dimensions of 40 mm×40 mm×2 mm, wereprepared according to the following steps and control procedures:

I. surface treatment with corindone sand+pickling in 20% HCl for 30minutes;

II. application of optional protective layers;

III. application of the surface electrocatalytic layer for oxygenevolution;

IV. electrochemical characterization tests (electrode potential) inelectrolytic media simulating industrial process working conditions;

V. comparison with reference samples prepared according to prior arttechnologies.

EXAMPLE 1

27 reference samples have been prepared according to the prior artteachings. The titanium matrix was pre-treated as described above (stepI). Then, 9 samples, identified as A, were activated with a surfacecoating based on Ta-Ir (65% by weight) (step III only); 9 samples,identified as B, were activated with an interlayer based on Ti-Ta (49%by weight) (step II) and, subsequently, with a surface coating of Ta-Ir(65% by weight) (step III) and 9 samples, identified as C were activatedwith an interlayer based on Ti-Ta (44% by weight)-Ir (12% by weight)(step II) and, subsequently, with a surface coating of Ta-Ir (65% byweight) (step III).

The compositions of the paints, interlayers and surface coatings arereported herebelow:

    __________________________________________________________________________    Paints for the interlayers                                                                            Paints for the surface coatings                         Components                                                                              mg/ml as metal                                                                            Components                                                                              mg/ml as metal                              __________________________________________________________________________    A =         =           TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                                                        50(Ta) 90(Ir)                               B TiCl.sub.3 TaCl.sub.5 HCl                                                               5,33(Ti) 5,03(Ta)                                                                         TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                                                        50(Ta) 90(Ir)                               C TiCl.sub.3 TaCl.sub.5 IrCl.sub.3 HCl                                                    5,00(Ti) 5,00(Ta) 1,36(Ir)                                                                TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                                                        50(Ta) 90(Ir)                               __________________________________________________________________________    Interlayers            Surface coatings                                                  % by weight                                                                          g/m.sup.2 as                                                                             % by weight                                                                         g/m.sup.2 as noble                           Components                                                                             as metal                                                                             total metal                                                                        Components                                                                          as metal                                                                            metal                                      __________________________________________________________________________    A =        =      =    Ta.sub.2 O.sub.5 --IrO.sub.2                                                        35(Ta) 65(Ir)                                                                       10                                         B Ta.sub.2 O.sub.5 --TiO.sub.2                                                           50(Ta) 50(Ti)                                                                        1    Ta.sub.2 O.sub.5 --IrO.sub.2                                                        35(Ta) 65(Ir)                                                                       10                                         C Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                                               44(Ta) 44(Ti)                                                                        2    Ta.sub.2 O.sub.5 --IrO.sub.2                                                        35(Ta) 65(Ir)                                                                       10                                                    12(Ir)                                                             __________________________________________________________________________

As regards the formation of the interlayer and surface coating, thepaint was applied by brushing or equivalent technique. This procedurewas repeated as many times as necessary to obtain the desired quantityof deposited metal. Between an interlayer and the other layer of appliedpaint, drying was carried out at 150° C., followed by thermaldecomposition in oven under forced air ventilation at 500° C. for 10-15minutes and subsequent natural cooling at ambient temperature.

EXAMPLE 2

18 samples made of titanium were prepared according to the inventionfollowing the procedures described above. The compositions of theinterlayer and surface coatings are illustrated in Table 2.1.

                                      TABLE 2.1                                   __________________________________________________________________________    Interlayer                   Surface coatings                                 Sample        % by weight                                                                             g/m.sup.2 as      % by weight   g/m.sup.2 as                                                                  noble                 code Components                                                                             as metal  total metal                                                                        Components   as metal      metal                 __________________________________________________________________________    2.1 a,b,c                                                                          Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                                               44(Ta) 44((Ti) 12(Ir)                                                                   2    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                                   30(Ta) 65(Ir)                                                                               10Ru                  2.2 a,b,c                                                                          "        "         "    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                                   35(Ta) 50(Ir)                                                                               "5(Ru)                2.3 a,b,c                                                                          "        "         "    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                                   35(Ta) 32.5(Ir)                                                                             "2.5(Ru)              2.4 a,b,c                                                                          "        "         "    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                                   35(Ta) 15(Ir)                                                                               "0(Ru)                2.5 a,b,c                                                                          "        "         "    Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                                      --RuO.sub.2  17.5(Ta) 12.5(Ti) 35(Ir)                                                                    "5(Ru)                2.6 a,b,c                                                                          "        "         "    Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                                      --RuO.sub.2  20(Ta) 10(Ti) 60(Ir)                                                                        "0(Ru)                __________________________________________________________________________

The interlayers and surface coatings of Table 2.1 were obtained bythermal treatment starting from paints containing precursors asdescribed in Table 2.2.

                  TABLE 2.2                                                       ______________________________________                                        Composition of the paints used for obtaining the                              interlayers and surface coatings                                              Sample Interlayer       Surface coating                                       code   Components                                                                              mg/ml as metal                                                                           Components                                                                            mg/ml as metal                            ______________________________________                                        2.1 a,b,c                                                                            TiCl.sub.3                                                                              5.00       TaCl.sub.5                                                                            39                                               TaCl.sub.5                                                                              5.00       IrCl.sub.3                                                                            85                                               IrCl.sub.3                                                                              1.36       RuCl.sub.3                                                                            6.5                                              HCl       110        HCl     110                                       2.2 a,b,c                                                                            "                    TaCl.sub.5                                                                            45.5                                                                  IrCl.sub.3                                                                            65                                                                    RuCl.sub.3                                                                            19.5                                                                  HCl     110                                       2,3 a,b,c                                                                            "                    TaCl.sub.5                                                                            45.5                                                                  IrCl.sub.3                                                                            42.3                                                                  RuCl.sub.3                                                                            42.3                                                                  HCl     110                                       2,4 a,b,c                                                                            "                    TaCl.sub.5                                                                            45.5                                                                  IrCl.sub.3                                                                            19.5                                                                  RuCl.sub.3                                                                            65                                                                    HCl     110                                       2.5 a,b,c                                                                            "                    TaCl.sub.5                                                                            20                                                                    TiCl.sub.3                                                                            14.3                                                                  IrCl.sub.3                                                                            40                                                                    RuCl.sub.3                                                                            40                                                                    HCl     110                                       2.6 a,b,c                                                                            "                    TaCl.sub.5                                                                            22.9                                                                  TiCl.sub.3                                                                            11.4                                                                  IrCl.sub.3                                                                            69                                                                    RuCl.sub.3                                                                            11.4                                                                  HCl     110                                       ______________________________________                                    

The samples thus prepared were subjected to electrochemical anodiccharacterization in three types of electrolytes, each one simulatingindustrial operating conditions as shown in Table 2.3.

                                      TABLE 2.3                                   __________________________________________________________________________    Electrochemical characterization: description of the tests.                   Test                                                                             Samples   Operating Conditions                                                                              Simulated industrial                         code                                                                             Sample code                                                                             Electrolyte                                                                             Operating parameters                                                                    process                                      __________________________________________________________________________    M  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 2.1a → 2.6a                                                                 F.sup.-                                                                           50 ppm                                                                              40° C.                                                                           (above 90% of the                               references: A1,B1,C1                                                                    Mn.sup.2+                                                                         5 g/l           worldwide electrolytic                                                        production)                                  N  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 2.1b → 2.6b                                                                 F.sup.-                                                                           5 ppm 40° C.                                                                           (the remaining 10% of                           references: A2,B2,C2                                                                    Mn.sup.2+                                                                         5 g/l           the worldwide electro-                                                        lytic production)                            O  present invention:                                                                      Na.sub.2 SO.sub.4                                                                 100 g/l                                                                             500 A/m.sup.2                                                                           cobalt                                          from 2.1c → 2.6c                                                                 H.sub.2 SO.sub.4                                                                  (pH = 2-3)                                                                          40° C.                                             references: A3,B3,C3                                                                    Mn.sup.2+                                                                         20 g/l                                                       __________________________________________________________________________

The electrochemical characterization comprised the determination of theelectrode potential as a function of the working time (expressed in thenormal hydrogen reference electrode scale as Volt (NHE)) and visualinspection of the sample at the end of the test.

The results obtained are summarized in table

                  TABLE 2.4                                                       ______________________________________                                        Electrochemical characterization: Experimental results.                                                      Morphological                                  Test Sample  Potential (V(NHE))                                                                              observations                                   code code    initial                                                                              100 h                                                                              1000 h                                                                              3000 h                                                                              at the end of the test                   ______________________________________                                             2.1a    1.70   1.72 1.90  ≧3.0                                                                         MnO.sub.2 compact deposit                     2.2a    1.68   1.70 1.95  ≧2.5                                                                         "                                             2.3a    1.65   1.68 1.90  ≧2.2                                                                         "                                             2.4a    1.62   1.75 ≧2.5 "                                             2.5a    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:                                                   spontaneous removal                           2.6a    1.68   1.72 1.74  1.75  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A1      1.69   1.85 2.10  ≧3.0                                                                         MnO.sub.2 compact deposit                     B1      1.72   1.82 2.10  ≧3.0                                                                         "                                             C1      1.72   1.70 1.95  ≧3.0                                                                         "                                        N    2.1b    1.65   1.70 1.90  ≧2.5                                                                         MnO.sub.2 compact deposit                     2.2b    1.63   1.66 1.85  ≧2.2                                                                         "                                             2.3b    1.60   1.62 1.80  ≧2.0                                                                         "                                             2.4b    1.58   1.70 ≧2.0 "                                             2.5b    1.62   1.64 1.65  1.65  MnO.sub.2 partial coverage:                                                   spontaneous removal                           2.6b    1.64   1.65 1169  1.69  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A2      1.65   1.72 2.00  ≧2.8                                                                         MnO.sub.2 compact deposit                     B2      1.69   1.80 2.11  ≧3.0                                                                         "                                             C2      1.68   1.70 1.90  ≧2.5                                                                         "                                        O    2.1c    1.80   1.85 2.10  ≧3.0                                                                         MnO.sub.2 compact deposit                     2.2c    1.76   1.78 2.00  ≧2.5                                                                         "                                             2.3c    1.75   1.74 1.90  ≧2.2                                                                         "                                             2.4c    1.70   1.72 ≧4.00                                                                              "                                             2.5c    1.72   1.74 1.70  1.75  MnO.sub.2 partial coverage:                                                   spontaneous removal                           2.6c    1.74   1.75 1.77  1.80  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A3      1.80   1.95 ≧2.2 MnO.sub.2 compact deposit                     B3      1.84   1.95 ≧203 "                                             C3      1.78   1.90 ≧2.3 "                                        ______________________________________                                    

The analysis of the experimental data leads to the followingobservations:

the prior art coatings are irreversibly passivated by the manganesepresent in the electrolyte, after about 1000 hours of operation insimulated industrial conditions;

the presence of ruthenium in the electrocatalytic surface coatingtogether with iridium and tantalum improves the behaviour of theelectrode with respect to manganese without however eliminating theinconveniences. In fact, only a delay with time of the passivationphenomena is experienced, delay which depends on the ruthenium contentin the active layer. In particular, an optimum concentration (=35%) isobserved, which corresponds to longer lifetimes;

the concurrent presence of ruthenium and titanium in the surface coatingtogether with iridium and tantalum permits to obtain an electrochemicalsystem durable with time and not passivated by manganese.

EXAMPLE 3

Following the same procedures described above, 18 samples made oftitanium were prepared with a second type of surface coating of theinvention containing ruthenium, iridium, titanium and tantalum as majorcomponents, (for a total of 90-95%), cobalt and tin as minor components(for a total of 5-10% max.). The compositions of the interlayers andsurface coatings are reported in Table

                                      TABLE 3.1                                   __________________________________________________________________________    Interlayers                                                                                          g/m.sup.2                                                                         Coatings                                           Sample        % by weight                                                                            as total              % by weight  g/m as no-          code Components                                                                             as metal metal                                                                             Components        as metal     ble                 __________________________________________________________________________                                                              metal               3.1 a,b,c                                                                          Ta.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2                                                 44(Ta) 44(Ti) 12(Ir)                                                                   2   Ta.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2                                        --RuO.sub.2 CoO.sub.x                                                                           17.5(Ta) 17.5(Ti)                                                                          10                                                               32(Ir) 32(Ru) I(Co)              3.2 a,b,c                                                                          "        "        `   "                 17.5(Ta) 17.5(Ti)                                                                          "                                                                31.25(Ir) 31.25(Ru) 2.5(Co)      3.3 a,b,c                                                                          "        "        "   "                 17.5(Ta) 17.5(Ti)                                                                          "                                                                30(Ir) 30(Ru) 5(Co)              3.4 a,b,c                                                                          "        "        "   -                 17.5(Ta) 17.5(Ti)                                                                          "                                                                27.5(Ir) 27.5(Ru) 10(Co)         3.5 a,b,c                                                                          "        "        "   Ta.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2                                        --RuO.sub.2 CoO.sub.x SnO.sub.x                                                                 15(Ta) 10(Ti)                                                                              "5(Ir)                                                           35(Ru) 2.5(Co)                                                                2.5(Sn)                          3.6 a,b,c                                                                          "        "        "   "                 15(Ta) 10(Ti)                                                                              "                                                                33.75(Ir) 33.75(Ru)                                                           2.5(Co) 5(Sn)                    __________________________________________________________________________

The interlayers and surface coatings of Table 3.1 have been obtained bythermal treatment starting from paints of precursor salts as illustratedin Table 3.2.

                  TABLE 3.2                                                       ______________________________________                                        Composition of the paints used for obtaining the                              interlayers and surface coatings                                              Sample Interlayer       Surface Coating                                       code   Components                                                                              mg/ml as metal                                                                           Components                                                                            mg/ml as metal                            ______________________________________                                        3.1.a,b,c                                                                            TiCl.sub.3                                                                              5.00       TaCl.sub.5                                                                            24.2                                             TaCl.sub.5                                                                              5.00       TiCl.sub.3                                                                            24.2                                             IrCl.sub.3                                                                              1.36       IrCl.sub.3                                                                            45                                               HCl       110        RuCl.sub.3                                                                            45                                                                    CoCl.sub.2                                                                            1.4                                                                   HCl     110                                       3.2.a,b,c                                                                            "         "          TaCl.sub.5                                                                            25.2                                                                  TiCl.sub.3                                                                            25.2                                                                  IrCl.sub.3                                                                            45                                                                    RuCl.sub.3                                                                            45                                                                    CoCl.sub.2                                                                            3.6                                                                   HCl     110                                       3.3.a,b,c                                                                            "         "          TaCl.sub.5                                                                            26.3                                                                  TiCl.sub.3                                                                            26.3                                                                  IrCl.sub.3                                                                            45                                                                    RuCl.sub.3                                                                            45                                                                    CoCl.sub.2                                                                            7.5                                                                   HCl     110                                       3.4.a,b,c                                                                            "         "          TaCl.sub.5                                                                            25.5                                                                  TiCl.sub.3                                                                            25.5                                                                  IrCl.sub.3                                                                            40                                                                    RuCl.sub.3                                                                            40                                                                    CoCl.sub.2                                                                            14.5                                                                  HCl     110                                       3.5.a,b,c                                                                            "         "          TaCl.sub.5                                                                            17.1                                                                  TiCl.sub.3                                                                            11.4                                                                  IrCl.sub.3                                                                            40                                                                    RuCl.sub.3                                                                            40                                                                    CoCl.sub.2                                                                            2.8                                                                   SnCl.sub.4                                                                            2.8                                                                   HCl     110                                       3.6.a,b,c                                                                            "         "          TaCl.sub.5                                                                            17.3                                                                  TiCl.sub.3                                                                            11.5                                                                  IrCl.sub.3                                                                            38.9                                                                  RuCl.sub.3                                                                            38.9                                                                  CoCl.sub.2                                                                            2.9                                                                   SnCl.sub.4                                                                            5.7                                                                   HCl     110                                       ______________________________________                                    

The samples thus prepared have been subjected to anodic electrochemicalcharacterization in 3 types of electrolyte, each one simulatingindustrial operating conditions as shown in Table 3.3.

                                      TABLE 3.3                                   __________________________________________________________________________    Electrochemical characterization: description of the tests.                   Test                                                                             Sampling  Operating Conditions                                                                              Simulated Industrial                         code                                                                             Sample code                                                                             Electrolyte                                                                             Operating parameters                                                                    Process                                      __________________________________________________________________________    M  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 3.1a → 3.6a                                                                 F.sup.-                                                                           50 ppm                                                                              40° C.                                                                           (above 90% of the                               references: A4,B4,C4                                                                    Mn.sup.2+                                                                         5 g/l           worldwide electrolytic                                                        production)                                  N  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 3.1b → 3.6b                                                                 F.sup.-                                                                           5 ppm 40° C.                                                                           (the remaining 10% of                           references: A5,B5,C5                                                                    Mn.sup.2+                                                                         5 g/l           the worldwide electro-                                                        lytic production)                            O  present invention:                                                                      Na.sub.2 SO.sub.4                                                                 100 g/l                                                                             500 A/m.sup.2                                                                           cobalt                                          from 3.1c → 3.6c                                                                 H.sub.2 SO.sub.4                                                                  (pH = 2-3)                                                                          40° C.                                             references: A6,B6,C6                                                                    Mn.sup.2+                                                                         20 g/l                                                       __________________________________________________________________________

The characterization comprised the determination of the electrodepotential as a function of the working time and visual inspection of thesample at the end of the test.

The results obtained are summarized in table 3.4.

                  TABLE 3.4                                                       ______________________________________                                        Electrochemical characterization: Experimental results.                                                      Morphological                                  Test Sample  Potential (V(NHE))                                                                              observations                                   code code    initial                                                                              100 h                                                                              1000 h                                                                              3000 h                                                                              at the end of the test                   ______________________________________                                        M    3.1a    1.65   1.65 1.68  1.72  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.2a    1.64   1.65 1.67  1.68  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.3a    1.60   1.63 1.65  1.69  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.4a    1.58   1.62 1.65  1.65  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.5a    1.62   1.60 1.55  1.58  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.6a    1.64   1.62 1.64  1.68  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A4      1.69   1.85 2.20  ≧3.0                                                                         MnO.sub.2 compact deposit                     B4      1.72   1.80 1.95  ≧3.0                                                                         "                                             C4      1.68   1.75 1.90  ≧3.0                                                                         "                                        N    3.1b    1.60   1.62 1.60  1.64  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.2b    1.62   1.60 1.62  1.70  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.3b    1.58   1.60 1.62  1.65  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.4b    1.55   1.58 1.65  1.75  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.5b    1.60   1.62 1.58  1.63  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.6b    1.62   1.64 1.70  1.74  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A5      1.65   1.80 2.20  ≧2.8                                                                         MnO.sub.2 compact deposit                     B5      1.70   1.75 1.90  ≧3.0                                                                         "                                             C5      1.65   1.70 1.90  ≧2.5                                                                         "                                        O    3.1c    1.75   1.77 1.77  1.80  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.2c    1.72   1.72 1.74  1.75  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.3c    1.68   1.64 1.68  1.70  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.4c    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.5c    1.70   1.68 1.70  1.72  MnO.sub.2 partial coverage:                                                   spontaneous removal                           3.6c    1.65   1.67 1.68  1.70  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A6      1.80   2.0  ≧2.3 MnO.sub.2 compact deposit                     B6      1.85   2.1  ≧2.4 "                                             C6      1.75   1.90 ≧2.3 "                                        ______________________________________                                    

The analysis of the data of table 3.4 leads to the followingobservations:

the prior art coatings are irreversibly passivated by the manganesepresent in the electrolyte after about 1000 hours of operation atsimulated industrial conditions;

the presence of cobalt, in the system comprising ruthenium, iridium,tantalum (already examined in previous Example 2) further decreases theelectrode potential, mainly at the beginning of the operation;

the concurrent presence of cobalt and tin in the above system not onlydecreases the initial electrode potential but furthermore causes itsstabilization with time.

EXAMPLE 4

6 samples made of titanium have been prepared following theaforementioned procedure, without any interlayer, with the 4- or6-component surface coatings selected among the best from the tests ofthe previous examples. The compositions of the surface coatings aregiven in Table 4.1.

                                      TABLE 4.1                                   __________________________________________________________________________    Interlayers           Coatings                                                Sample     % by weight                                                                         g/m.sup.2 as                                                                              % by weight                                                                           g/m as no-                               code Components                                                                          as metal                                                                            total metal                                                                        Components                                                                           as metal                                                                              ble metal                                __________________________________________________________________________    4.1 a,b,c                                                                          /     /     /    Ta.sub.2 O.sub.5 --TiO.sub.2                                                         17.5(Ta) 12.5(Ti)                                                                     10                                                             IrO.sub.2 --RuO.sub.2                                                                35(Ir) 35(Ru)                                    4.2 a,b,c                                                                          /     /     /    Ta.sub.2 O.sub.5 --TiO.sub.2                                                         12.5(Ta) 12.5(Ti)                                                      IrO.sub.2 --RuO.sub.2                                                                35(Ir) 35(Ru)                                                          CoO.sub.x --SnO.sub.x                                                                2.5(Co) 2.5(Sn)                                  __________________________________________________________________________

The surface coatings of Table 4.1 were obtained by thermal treatmentfrom paints of precursor salts as shown in Table 4.2.

                  TABLE 4.2                                                       ______________________________________                                        Compositions of the paints used for preparing the surface                     coatings of Table 4.1                                                         Sample code    Components                                                                              mg/ml                                                ______________________________________                                        4.1.a,b,c      TaCl.sub.5                                                                              17.1                                                                TiCl.sub.3                                                                              17.1                                                                IrCl.sub.3                                                                              40                                                                  RuCl.sub.3                                                                              40                                                                  HCl       110                                                  4.2.a,b,c      TaCl.sub.5                                                                              14.3                                                                TiCl.sub.3                                                                              14.3                                                                IrCl.sub.3                                                                              40                                                                  RuCl.sub.3                                                                              40                                                                  CoCl.sub.2                                                                              2.9                                                                 SnCl.sub.4                                                                              2.9                                                                 HCl       110                                                  ______________________________________                                    

The samples thus prepared were subjected to anodic electrochemicalcharacterization in 3 types of electrolytes, each one simulating theindustrial operating as shown in table 4.3.

                                      TABLE 4.3                                   __________________________________________________________________________    Electrochemical characterization: description of the tests.                   Test                                                                             Sampling  Operating Conditions                                                                              Simulated industrial                         code                                                                             Sample code                                                                             Electrolyte                                                                             Operating parameters                                                                    process                                      __________________________________________________________________________    M  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 4.1a → 4.2a                                                                 F.sup.-                                                                           50 ppm                                                                              40° C.                                                                           (above 90% of the                               2.5a (Example 2),                                                                       Mn.sup.2+                                                                         5 g/l           worldwide electrolytic                          3.5a (Example 3),             production)                                     references:                                                                   A7,B7,C7.                                                                  N  present invention:                                                                      H.sub.2 SO.sub.4                                                                  150 g/l                                                                             500 A/m.sup.2                                                                           zinc                                            from 4.1b → 4.2b                                                                 F.sup.-                                                                           5 ppm 40° C.                                                                           (the remaining 10% of                           2.5b (Example 2),                                                                       Mn.sup.2+                                                                         5 g/l           the worldwide electro-                          3.5b (Example 3),             lytic production)                               references:                                                                   A8,B8,C8.                                                                  O  present invention:                                                                      Na.sub.2 SO.sub.4                                                                 100 g/l                                                                             500 A/m.sup.2                                                                           cobalt                                          from 4.1c → 4.2c                                                                 H.sub.2 SO.sub.4                                                                  (pH = 2-3)                                                                          40° C.                                             2.5c (Example 2),                                                                       Mn.sup.2+                                                                         20 g/l                                                          3.5c (Example 3),                                                             references:                                                                   A9,B9,C9.                                                                  __________________________________________________________________________

The characterization comprising the determination of the electrodepotential as a function of the working time and visual inspection of thesample at the end of the test, gave the experimental results summarizedin 4.4.

                  TABLE 4.4                                                       ______________________________________                                        Electrochemical characterization: Experimental results.                                                      Morphological                                  Test Sample  Potential (V(NHE))                                                                              observations                                   code code    initial                                                                              100 h                                                                              1000 h                                                                              3000 h                                                                              at the end of the test                   ______________________________________                                        M    4.1a    1.67   1.68 1.70  1.74  MnO.sub.2 partial coverage:                                                   spontaneous removal                           4.2a    1.66   1.68 1.67  1.70  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A7      1.69   1.85 2.20  ≧3.0                                                                         MnO.sub.2 compact deposit                     B7      1.72   1.80 2.20  ≧3.0                                                                         "                                             C7      1.68   1.75 1.90  ≧3.0                                                                         "                                             2.5a    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 2)                                                                        3.5a    1.62   1.60 1.55  1.58  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 3)                                                                   N    4.1b    1.67   1.70 1.70  1.74  MnO.sub.2 partial coverage:                                                   spontaneous removal                           4.2b    1.65   1.68 1.72  1.70  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A8      1.65   1.80 2.20  ≧2.8                                                                         MnO.sub.2 partial coverage:                                                   spontaneous removal                           B8      1.70   1.75 1.90  ≧3.0                                                                         MnO.sub.2 partial coverage:                                                   spontaneous removal                           C8      1.65   1.70 1.90  ≧2.5                                                                         MnO.sub.2 partial coverage:                                                   spontaneous removal                           2.5b    1.62   1.64 1.65  1.65  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 2)                                                                        3.5b    1.60   1.62 1.58  1.63  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 3)                                                                   O    4.1c    1.78   1.75 1.80  1.80  MnO.sub.2 partial coverage:                                                   spontaneous removal                           4.2c    1.74   1.70 1.75  1.78  MnO.sub.2 partial coverage:                                                   spontaneous removal                           A9      1.80   2.00 ≧2.20                                                                              MnO.sub.2 compact deposit                     B9      1.85   2.10 ≧2.30                                                                              "                                             C9      1.75   1.90 ≧2.30                                                                              "                                             2.5c    1.72   1.74 1.70  1.75  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 2)                                                                        3.5c    1.70   1.68 1.70  1.72  MnO.sub.2 partial coverage:                   (Exam-                          spontaneous removal                           ple 3)                                                                   ______________________________________                                    

From the analysis of the experimental results it is possible to make thefollowing observations:

the prior art coatings are irreversibly passivated by manganese presentin the electrolyte after about 1000 hour of simulated industrialconditions.

the coatings of the present invention, without any interlayer, althoughoperating at slightly higher potentials with respect to those typical ofanodes provided with the interlayer are equally stable to fluorides andare not passivated by manganese.

What is claimed is:
 1. An anode for oxygen evolution in acidelectrolytes containing sulfuric acid and high quantities of manganeseand optionally fluorides in small quantities, said anode comprising atitanium matrix provided with a surface electrocatalytic coating,wherein said surface electrocatalytic coating consists of pure oxides ormixed oxides of the metals of the group consisting of titanium,tantalum, tin, cobalt, ruthenium and iridium, wherein ruthenium andiridium are major components, cobalt and tin are minor components andtitanium and tantalum are components present in intermediate quantities.2. The anode of claim 1 wherein ruthenium and iridium are present as atotal by weight comprised between 30 and 90%, titanium and tantalum arepresent as a total by weight between 30 and 40%, cobalt and tin arepresent as a total by weight comprised between 1 and 10%.
 3. The anodeof claim 2 wherein the amount of cobalt and tin is 4 to 6%.
 4. The anodeof claim 1 characterized in that it further comprises a conductiveinterlayer between said matrix and said electrocatalytic coating, havingthe function of protection against fluorides.
 5. A method for preparingthe anode of claim 1, characterized in that it comprises the followingsteps:corindone sandblasting of the titanium matrix, pickling inhydrochloric acid, optionally formation of a protective interlayer byapplying paints containing thermally decomposable compounds of themetals of the platinum group, and metals of the groups IV B and V B,with drying and thermal decomposition in air, with the repetition of thesteps of application, drying, decomposition up to obtaining a desiredthickness, formation of the surface electrocatalytic coating by applyingpaints containing thermally decomposable compounds of at least one noblemetal selected from the group of ruthenium and iridium, at least onevalve metal selected from the group of titanium and tantalum and atleast one non-noble metal selected from the group of cobalt and tin,with drying and thermal decomposition in air, with the repetition of thesteps of application, drying, decomposition up to obtaining the desiredthickness.
 6. The process of claim 5 wherein the platinum group metal isiridium and the metals of group IV B and V B are titanium and tantalumand the non-noble metal is tin and/or cobalt.
 7. In the process ofelectrodepositing a metal from an aqueous solution containing ions ofsaid metal, magnesium ions and optionally fluoride ions by electrolysisof the solution between an anode and a cathode, the improvementcomprising using an anode of claim
 1. 8. The method of claim 7 whereinthe ions are of zinc or cobalt.